The research proposed will deal principally with three questions: 1) Is the decreased mitogenic response of T lymphocytes from aged animals due to a T cell defect or is it due to the deficiency in macrophages and 2) do hydrolytic enzymes which have mitogenic effects on beta-lymphocytes work equally well on cells from young and aged animals, (3) can differences qualitative or quantitative, in T cell surface proteins from young and aged animals be documented. Answers to these questions should provide some insight into the decline of immunologic capacity with age.